The invention relates generally to sensors for monitoring the partial pressure of oxygen in various environments and more specifically relates to fiber-optic devices for monitoring the partial pressure of oxygen in medical applications.
Various amperometric electrochemical methods exist for measuring the partial pressure of oxygen. However, these methods are generally unsatisfactory for in vivo applications due to severe and unpredictable drift related to difficulties associated with the fabrication of microsensors and with membrane contamination. As used in this context, membrane contamination refers to clogging or fouling of the membrane and is highly undesirable because it can cause such devices to produce incorrect readings for the partial pressure of oxygen.
An alternative to the use of electrochemical sensors for in vivo applications is the use of optically based oxygen sensors. Several oxygen-based sensor systems have been described previously. For example, U.S. Pat. No. 4,003,707 issued Jan. 18, 1977, to Lubbers describes the idea of using quenching by oxygen of the fluorescence of pyrene dibutyric acid dissolved in dimethyl formamide with the solution enclosed in a gas-permeable membrane.
One problem, however, with the device described in the patent to Lubbers is that such devices are difficult to fabricate as miniature devices for use in medical applications.
Another optical device is described in Peterson, Analyt. Chem., 56, 62-67 (1984). Peterson describes the use of a two-fiber optical cable having a sensing tip consisting of perylene dibutyrate absorbed on a powdered polystyrene support and enclosed in a gas permeable membrane. The dye is excited by light sent down one of the fibers. The resulting fluorescence is detected with the other fiber. Quenching of the fluorescence of perylene dibutyrate by oxygen is again used in this method.
Another general type of optical device for monitoring the partial pressure of oxygen can be based on the use of ruthenium (II) complexes as luminescent sensors. The properties of such complexes are described in Klassen et al., "Spectroscopic Studes of Ruthenium (II) Complexes. Assignment of the Luminescence", The Journal of Chemical Physics, Vol. 48, No. 4, (1968), Pages 1853-1858, and in Demas et al., "Energy Transfer from Luminescent Transition Metal Complexes to Oxygen", Journal of the American Chemical Society, Vol. 99, No. 11, (1977), Pages 3547-3551.
The use of perylene dibutyrate or pyrene dibutyric acid mounted on a solid support, or in solution, and enclosed in a membrane is unsatisfactory because of the complexity of fabrication and the poor sensitivity of the dyes. The luminescence of these dyes change substantially less than twofold when the partial pressure of oxygen changes from 0 to 760 mm. Hg. These changes have been measured and found to be only about ten percent or less. The ruthenium complex is much more sensitive than the other two materials, but is very slow to respond when used in the unplasticized polyvinyl chloride (PVC) or silicone rubber systems described by Demas and Bacon.
U.S. Pat. Nos. 4,399,099 and 4,321,057 to Buckles describes an oxygen sensor made by coating an optical fiber core with a cladding material which interacts with oxygen thereby changing the amount of transmitted light. His method requires that both ends of the fiber be accessible so that effectively two fibers (i.e., two fiber ends) are required for a given sensor if used in a catheter application.
The use of two fibers and/or a remote light source appears to be a requirement for most optical sensing devices for monitoring the partial pressure of oxygen. This makes the use of such devices impractical in remote sensing applications where only a single fiber is available, and where that single fiber must serve as both an excitation source and a signal conduit, particularly where space constraints exist.
Therefore, a need exists to provide a miniaturized sensing system, for monitoring the partial pressure of oxygen, which is easily fabricated, is sufficiently responsive to small changes in the partial pressure of oxygen, can be operated using a single optical fiber in remote applications, and is not subject to the effects of membrane contamination such as deterioration of the accuracy of the sensor.